1. Field of the Invention
This invention relates in general to shock absorbers, and in particular to shock absorbers for wireline instruments.
2. Description of the Prior Art
Survey instruments of various types are sometimes lowered into oil well boreholes on a wireline in order to record various downhole conditions. As an instrument is being lowered into a wellbore, the instrument will sometimes strike a ledge or the bottom of the wellbore. Because of the fragile nature of some survey instruments, it is necessary to protect such instruments from impact shocks.
A shock absorber may be defined as a device which produces a dissipative (non-recoverable) force over a given displacement to absorb energy and to remove the energy from a system. The most common type of shock absorber used on wireline instruments is a mechanical spring, mounted near the lower end of the instrument. This type of shock absorber has a disadvantage in that it tends to recoil.
U.S. Pat. No. 3,653,468 (Marshall) shows a wireline instrument shock absorber which diminishes or eliminates recoil. The shock absorber has washer-like projections, which extend radially outward from a hollow cylinder. A cutter bar slides over the hollow cylinder and engages the washer-like projections. Upon impact, kinetic energy is expended by the cutter bar shearing off some of the washer-like projects.
U.S. Pat. No. 3,779,591 (Rands) shows a shock absorbing system for use on an automobile safety bumper. This system contains a shaft and a telescoping sleeve, and connects the bumper to the vehicle structure. In a collision, the shaft is telescoped into the sleeve, and longitudinal strips of material are sheared off of the shaft by the cutting edges of the sleeve.